Transposase-derived proteins FHY3/FAR1 interact with PHYTOCHROME-INTERACTING FACTOR 1 to regulate chlorophyll biosynthesis
Weijiang Tang, Wanqing Wang, Dongqin Chen, Qiang Ji, Yanjun Jing, and Rongcheng Lin *

Key Laboratory of Photobiology, Institute of Botany, the Chinese Academy of Sciences, Beijing 100093, China.
*Corresponding Author：Tel: (+8610) 62836905; E-mail: rclin@ibcas.ac.cn

Abstract
Successful chlorophyll biosynthesis during initial light exposure is critical for plant survival and growth, as excess accumulation of chlorophyll precursors in darkness can cause photooxidative damage to cells. Therefore, efficient mechanisms have evolved to precisely regulate chlorophyll biosynthesis in plants. Here, we identify FAR-RED ELONGATED HYPOCOTYL1 (FHY3) and FAR-RED IMPAIRED RESPONSE1 (FAR1), two transposase-derived transcription factors, as positive regulators of chlorophyll biosynthesis in Arabidopsis thaliana. We show that null mutations in FHY3 and FAR1 cause reduced protochlorophyllide (a precursor of chlorophyll) levels in darkness and less photobleaching in the light. We find that FHY3 directly binds to the promoter and activates expression of HEMB1, which encodes 5-aminolevulinic acid dehydratase in the chlorophyll biosynthetic pathway. We reveal that PIF1 physically interacts with the DNA binding domain of FHY3, thereby partly repressing FHY3/FAR1-activated HEMB1 expression. Strikingly, FHY3 expression is up-regulated by white light. In addition, our genetic data indicate that overexpression, severe reduction, or lack of HEMB1 impairs plant growth and development. Together, our findings reveal a crucial role of FHY3/FAR1 in regulating chlorophyll biosynthesis, thus uncovering a new layer of regulation by which light promotes plant dark-light transition in early seedling development.

Key Words: transcription factor; chlorophyll biosynthesis